What's the Kell factor in 720p
 

What's the Kell factor in 720p?

michael

According to a brief Google search, some folks put it as low as 0.67 and others say it's as high as 0.9.

Do we have a lines of res on that? Isn't 1080i like in the 800+ ball park. What would 720p be?

See the ASTC DTV comparison chart at http://www.evansassoc.com/lib/Atsc-dtv.html -- they're assuming a Kell factor of 0.9 for all progressive modes including 720p.

0.67 comes from http://www.dtvforum.info/lofiversion....php/t531.html

For anybody who may be wondering what the heck we're talking about, see "The Kell Factor Explained" at http://members.aol.com/ajaynejr/kell.htm

Good Links! What I wanted exactly!

Thanks DVINFO Master............

I've got to re-read Poynton, but I think things are a bit more complex than that:

Interlaced video is filtered to stop line twitter, progressive, as true progressive for a true progressive display is not.

Both interlaced and progressive video are effected by the Kell factor, which relates to the perceived resolution of a display.

The "interlace factor" and the "Kell factor" are two different things, but it sounds like a lot of people lump them together. As far as I understand, Kell did his experiments with progressive video.

Anyway, let me re-read Poynton and sumarise back here....

Graeme

Don't forget the Nyquist limit. Aaaaggghhh! The Nyquist limit!

Oh there you go, you had to bring up seaquest..Oh oh I mean Nyquist.

Yeah Nyquist. Good band, right. Ohhh, Sh&* , I can't know everything.

What the hell are Nyquist's for gods sakes?

Michael, - not knowing

See "Sampling and Nyquist's Theorem for Audio and Video"

located at http://members.aol.com/ajaynejr/nyquist.htm

and then see also "Nyquist and TV Lines, what is the resolution of a digitized TV image"

located at http://www.spectra-one.com/digitalvideo.html#digtv

I too have wondered if the .7 Kell factor is multiplied by a the interlace filter factor.

So I created a model of interleave handing line pairs usind row-pair summation. Then used the created frames as the basis for Kell.

I've concluded Kell of .7 is applied equally to interlace and progressive V rez. The reasons P looks so much higher in V res I don't think is based upon two filter functions killing I V rez. Otherwise, NTSC V rez wouldn't be about 340-lines.

And, yes Kell is .7 done on P video.

Steve! Somehow I just *knew* this thread would attract you. And there really hasn't been any substance to it yet, until now! Heh.

My own understanding -- correct me if I'm wrong -- is that we have the Kell factor to thank for 720p looking as good as, or in some cases better than 1080i, but I often wonder if I'm not confusing Kell vs. the interlace factor.

I'm getting a headache- too much for a Sunday morning.. and to think, all these years, I've just used my eyeballs and trusted the messages I recieved from my brain!

no, seriously, thanks for the info, interesting!

Now that we have the factor, what is the scientific rez of 720p, approximate area?

It depends on what you're imaging. If you lined up perfectly, pixel-for-pixel, with a 1280x720 grid (on a piece of paper, for example), then 720p would yield 1280 x 720.

Interlaced video never will. The interlaced factor is always going to cost you about 30% of your resolution -- the interlaced factor meaning that fields are blended together to reduce flicker. So the very best you'll ever get out of a 1920x1080i system would be about 1920x800x30.

Not so with 720p. With 720p, every pixel stands alone, and is not de-interlaced or filtered. So you could, under ideal circumstances, get 1280x720x60.

When the Kell factor comes into play is when you haven't lined up exactly perfectly with the source material. When the grid on the paper lines up on the CCD perfectly, you will have perfect line-for-pixel reproduction on a progressive camera (although never on an interlaced camera). But when the grid lines and the CCD pixels don't line up perfectly, when lines straddle pixel boundaries, that's when the Kell factor comes into play. The Kell factor is an attempt to take into account the resolution that's lost when the pixels and lines aren't perfectly matched, so some gray area comes into play.

Thanks Barry!


Michael